Make NameSpecification and its element ExpressibleByStringLiteral

[natecook1000]

This simplifies declaring alternate names by allowing just a string representation of an option or flag name. The string is parsed to split on spaces, and then validates that each name has either a one or two dash prefix and is ASCII only.

Existing style:

@Flag(name: [.customLong("hex-output"), .customShort("x")])
var hexadecimalOutput = false

New style:

@Flag(name: "--hex-output -x")
var hexadecimalOutput = false

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[rgoldberg]

@natecook1000 Is there any spec for the valid characters for each possible part of a command line (including first characters; e.g., I assume that subcommands shouldn't start with a -)?

Are such restrictions programmatically enforced in a centralized place?

[natecook1000]

@rgoldberg No, we don't have any of that validation in place. There's a certain degree of self-enforcement in place, in that things will just not work properly if you do things like defining an option with a space in its name. But we should add some of that, and can be more stringent in this case, since it's a new behavior.

[rgoldberg]

@natecook1000 Thanks for the info.

The problem that I see with being more stringent in this case is, e.g., that someone could have an existing flag name that contravenes the new restrictions for this feature. One would assume that flag names should work everywhere or nowhere, not for some features but not for others.

I'm very much in favor of explicit character restrictions, but I think it's best to programmatically enforce them all in one central place & to document them, then to use & rely on those restrictions throughout.

I would assume that enacting the restrictions would require a new major version of SAP, in case now-invalid characters were used with the existing major version.